Epithets
| rivalis | growing by a stream or river |
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| rivularis | growing by a stream or river |
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| rivulus | growing by a stream or river |
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| rostrata | beaked |
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| rostratum | beaked |
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| rotata | wheel-like |
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| rotatus | wheel-like |
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| rotunda | round |
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| rotundata | rounded |
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| rotundus | round |
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| rubens | red |
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| ruber | red |
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| rubiginosa | rusty |
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| rubra | red |
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| rufa | reddish brown |
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| rufus | reddish brown |
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| rupestre | found near rocks |
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| rupestris | found near rocks |
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| saccharata | sugar, sweet |
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| saccharum | sugar, sweet |
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| sagittalis | arrow-like or -shaped |
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| sagittatus | arrow-like or -shaped |
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| sarmentosa | producing runners |
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| sarmentosum | producing runners |
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| sativa | cultivated |